A typical truck loading dock of a building includes an exterior doorway with an elevated platform for loading and unloading vehicles such as trucks and trailers. Many loading docks have a dock leveler to compensate for height differences between the loading dock platform and an adjacent bed of the truck or trailer. A typical dock leveler includes a deck, also known as a ramp or dockboard, which is pivotally hinged along a back edge to vary the height of a front edge. An extension plate, or lip, extends outward from the deck's front edge to span the gap between the rear of the trailer bed and the front edge of the deck. Extending from the deck's front edge, the lip rests upon the truck bed to form a bridge between the deck and the bed. This allows personnel and material handling equipment, such as a forklift truck, to readily move on and off the vehicle during loading and unloading operations.
When a forklift drives over the dock leveler and onto the trailer bed, the weight of the forklift and/or the cargo it may be carrying can add a significant load to the truck bed. Likewise, when the forklift exits the truck bed, weight is removed from the trailer. Thus, the load carried by the trailer changes repeatedly during the loading/unloading process. The trailer's suspension may respond to these load changes by allowing the trailer to raise and lower accordingly.
Unfortunately, the resulting vertical movement of the trailer can create some problems. For instance, the rear or side edges of the trailer usually engage some type of dock seal that is mounted at a generally fixed location along the doorway of the dock, so vertical movement of the trailer can wear out the seal. Also, a forklift suddenly descending upon entering the trailer can be disconcerting to the forklift driver. Such movement of the trailer becomes worse when the trailer has an air suspension system.
With air suspension, air-pressurized bladders support the weight of the trailer and its cargo. Air suspension systems typically include an air compressor, a holding tank, and various control valves that cooperate to add or release a controlled amount of air from the bladders to help maintain the trailer at a certain height. So, when a forklift enters the trailer, pressurized air is forced into the bladders to compensate for the forklift's added weight. Due to the suspension system's delayed response time, however, the trailer may initially sink or lower when the forklift first enters and later rise back up toward its intended height before the forklift departs. Then, when the forklift leaves and removes its weight from the trailer, the recently added air in the bladders lifts the trailer to an elevation that is greater than its designed height. The system compensates for the overshoot by releasing some air from the bladders until the trailer settles back down to its original height. This down/up cycle of the trailer repeats itself with every load the forklift takes on and/or off the trailer. Compared to other suspension systems, air suspensions usually provide much greater vertical movement.
To eliminate the repeated movement of the trailer, an air suspension system may simply dump or completely exhaust the air from the bladders before the loading or unloading process begins. This causes the trailer to descend until the suspension system bottoms out, whereby the suspension becomes inactive, and the trailer remains at its bottomed out position while the trailer is loaded or unloaded of its cargo. Although this may correct the problems associated with movement of the trailer during loading and unloading, the low position of the trailer bed can create another problem. For the dock leveler to reach such an extremely low trailer bed, the deck may need to be set at such a steep incline that it may be difficult for the forklift to travel across the deck.
Some loading docks may have a vehicle restraint that helps prevent a truck or trailer from accidentally pulling away from the dock. Such vehicle restraints usually include a hook or barrier that reaches up in front of the vehicle's RIG (rear impact guard) or ICC bar. Instead of inhibiting vertical movement of the vehicle during its loading or unloading, these vehicle restraints do just the opposite; they accommodate or allow the vehicle the freedom to move vertically. One such known restraint includes a spring that compresses with downward force exerted by an ICC bar. Similarly, the vehicle restraint includes a pressure relief valve that can be set to hold the weight of the restraint itself, but the relief valve is not meant to inhibit the downward movement of the vehicle.
In some examples, a solid, immovable support structure, such as a hydraulic jack, could be placed underneath the ICC bar to completely eliminate any vertical movement of the vehicle and/or actually lift the vehicle. However, such a support structure could result in an excessive upward reactive force being applied to the ICC bar and the underside of the trailer bed to which the bar is attached. More specifically, if the trailer bed were held stationary, any added weight of cargo and/or the weight of a forklift entering the trailer would be transmitted through the ICC bar and to the frame, neither of which may be designed to support such loads. Thus, holding the trailer bed completely immovable could damage the ICC bar and/or other parts of the trailer.